For motor vehicles, such as, for example, cars, trucks and buses, fuel cost represents a significant expense for the owner or user of the vehicle. For a haulage company, for example, apart from the cost of procurement of the vehicle, the main items of expenditure for the running of a vehicle are constituted by pay to the driver of the vehicle, costs of repairs and maintenance, and fuel for propulsion of the vehicle. The fuel cost can have a very large impact on profitability for the haulage company. Hence, a number of different systems have been developed in order to reduce fuel consumption, such as, for example, fuel-efficient engines and fuel-saving speed control devices.
FIG. 1 shows a schematic representation of a drive train in a vehicle 100. The drive train comprises an internal combustion engine 101, which is connected in a conventional manner, via an output shaft 102 from the internal combustion engine 101, usually via a flywheel, to an input shaft 109 of a gearbox 103 by means of a clutch 106. The clutch 106 can be constituted, for example, by an automatically controlled clutch, and is controlled by the control system of the vehicle via a control unit 700 (FIG. 7). The control unit 700 can also control the gearbox 103.
The gearbox 103 is here illustrated schematically as a unit. However the gearbox 103 can also physically consist of a plurality of interacting gearboxes, for example a range gearbox, a main gearbox and a split gearbox, which are arranged along the drive train of the vehicle. The gearbox can comprise a suitable number of gear positions. In contemporary gearboxes for heavy duty vehicles there are usually twelve forward gears, two reverse gears and a neutral gear position. If the gearbox 103 physically consists of a plurality of part gearboxes according to the above, these twelve forward gears are distributed amongst two gears in the range gearbox, three gears in the main gearbox and two gears in the split gearbox, which together constitute twelve gear positions (2×3×2=12). The vehicle 100 further comprises drive shafts 104, 105, which are connected to the drive wheels 110, 111 of the vehicle and are driven by an output shaft 107 from the gearbox 103 via an axle gearing 108, such as, for example, a conventional differential.
The vehicle 100 further comprises a variety of different braking systems, such as a conventional service braking system, which can comprise, for example, brake disks with associated brake linings (not shown) arranged next to each wheel. The engine 101 can be controlled on the basis of instructions from a speed control, in order to maintain a constant actual vehicle speed and/or vary the actual vehicle speed so that a fuel consumption which is optimized within reasonable speed limits is obtained. The engine 101 can also be controlled by a driver of the vehicle.
During the driving of a vehicle, the gear selection has a large influence on the fuel consumption, since the engine speed is directly dependent on this gear selection. Previously known solutions have had problems with determining at what point downshifts should be made, at the same time as they have sometimes led the vehicle to fall below a lowest permitted speed vmin. This has had the effect that the gear selection, due to drivability reasons, has not been able to be calibrated so as to achieve sufficiently low engine speed and thus to be fuel-efficient, since such calibration attempts for previously known solutions have led to belated downshifts and loss of speed.
In situations when a currently utilized transmission mode is operating at its maximum torque, the actual vehicle speed vact has traditionally been allowed to fall prior to the execution of a downshift to a lower transmission mode. For example, in an economy mode (“eco”) for the gear selection, the downshift has previously been delayed for quite a long time. This is a fuel-efficient way of driving the vehicle, since the engine speed is held down as far and as long as possible. One problem is, however, that on the hills where a downshift to a lower transmission mode must anyway be made, this manner of driving the vehicle is perceived by the driver as unintelligent and not intuitively pleasing. The result is that the previously known systems are at risk of gaining limited use.
In descents, for example, or in situations in which the vehicle must reduce its actual speed vact, fuel savings have historically been made by a reduced request for positive engine torque, alternatively with the aid of dragging. The reduced request for positive engine torque means that the driving force in the direction of travel, which the internal combustion engine delivers via the drive wheels, is reduced, for example by reduced fuel injection in the engine 101, thereby reducing the fuel consumption. Dragging means driving the vehicle with closed drive train, that is to say with the combustion engine 101 connected to the drive wheels 110, 111 of the vehicle, at the same time as the fuel supply to the internal combustion engine 101 is shut off.
One way of further lowering the fuel consumption is to coast the vehicle by, as described below, either utilizing a neutral gear position in the gearbox 103 or opening the clutch. Through the use of coasting, a still lower fuel consumption than with dragging is achieved, since engine braking is eliminated, at the same time as the engine speed is reduced to a minimum. Coasting can be realized with the engine 101 running or shut off. If the engine 101 is running, coasting is only profitable in a conventional vehicle if the vehicle is not braked or will not need to be braked.
Previously known solutions have had problems with determining at what point the coasting should be suspended in order to obtain a pliable and flexible function which is also imperceptible in terms of comfort, that is to say when it is time to, at an intuitively correct moment, engage a physical gear in the gearbox and/or close the clutch, as well as to determine when the coasting has to be suspended to prevent the vehicle from falling below the lowest permitted speed vmin.
Previously known solutions have thus chosen transmission modes in the vehicle in a manner which is neither optimal from the perspective of drivability nor comfort. The transmission modes here comprise coasting, as well as gear positions in the gearbox. This has led to low driver acceptance for low engine speed gear selection and coasting, which has led to a low level of use of the function.